Process for the production of a desulfurized gasoline from a gasoline fraction that contains conversion gasoline

ABSTRACT

Production of gasolines with low sulfur contents from a starting gasoline containing sulfur-containing compounds comprising a stage a) for selective hydrogenation of non-aromatic polyunsaturated compounds present in the starting gasoline, a stage b) for increasing the molecular weight of the light sulfur-containing products that are initially present in the gasoline that enters this stage, a stage c) for alkylation of at least a portion of the sulfur-containing compounds present in the product that originates stage b), a stage d) for fractionation of the gasoline that originates from stage c0 into at least two fractions, one fraction virtually lacking in sulfur-containing compounds, whereby the other contains a larger proportion of sulfur-containing compounds (heavy gasoline), a stage e) for catalytic treatment of the heavy gasoline for transformation of sulfur-containing compounds under conditions for the at least partial decomposition of hydrogenation of these sulfur-containing compounds.

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a scheme for desulfurization ofconversion gasolines and in particular gasolines that are obtained fromcatalytic cracking, fluidized-bed catalytic cracking (FCC), a coking (inEnglish) process, a visbreaking (in English) process, a pyrolysisprocess. This process can also treat direct distillation gasolines(straight run, in English) that are mixed with at least one of thegasolines cited above. The process of this invention makes it possibleto reach high desulfurization rates while limiting the octane loss dueto the saturation of the olefins that are observed during thehydrodesulfurization reactions. The invention relates to a process forthe production of gasoline with a low sulfur content comprising ahydrogenation, a stage for transformation of sulfur-containingcompounds, a stage for alkylation of sulfur-containing compounds thatare not transformed in the preceding stage, a fractionation into a lightfraction and into at least one heavy fraction and optionally one or moreintermediate fractions, and a hydrodesulfurization of a heavy fractionand/or an intermediate fraction. This process makes it possible toupgrade a gasoline fraction that optionally also comprises hydrocarbonswith two, three or four carbon atoms, by reducing the total sulfurcontent of said fraction to very low levels that are compatible with thecurrent or future specifications. This desulfurization is furthermorecarried out without an appreciable reduction of the gasoline yield andby minimizing the reduction of the octane number.

[0002] Prior Art

[0003] The production of reformulated gasolines that meet newenvironmental standards requires in particular that their olefinconcentration be reduced slightly but that their sulfur concentration bereduced to a significant extent. The conversion gasolines and moreparticularly the one that is obtained from catalytic cracking, which mayrepresent 30 to 50% of the gasoline pool, have high olefin and sulfurcontents. The sulfur that is present in the reformulated gasolines canbe nearly 90%, attributed to the catalytic cracking gasoline (FCC,“Fluid Catalytic Cracking,” or fluidized-bed catalytic cracking). Thedesulfurization (hydrodesulfurization) of gasolines and primarily FCCgasolines is therefore of obvious importance for achieving thespecifications.

[0004] The hydrodesulfurization of the feedstock that is sent tocatalytic cracking produces gasolines that typically contain 100 ppm ofsulfur. The hydrotreatment units of catalytic cracking feedstocksoperate, however, under severe temperature and pressure conditions,which assumes a significant hydrogen consumption and a high investment.In addition, all of the feedstock should be desulfurized, which involvesthe treatment of very large volumes of feedstock.

[0005] The hydrodesulfurization of the cracking gasolines, when it iscarried out under standard conditions that are known to one skilled inthe art, makes it possible to reduce the sulfur content of the fraction.This process, however, exhibits the major drawback of causing a verysignificant drop in the octane number of the fraction caused by thesaturation of all of the olefins during hydrotreatment.

[0006] The teaching of the prior art is extremely non-specific, and alarge number of solutions have been considered to obtain a desulfurizedgasoline that meets the standards that are currently in force and futurestandards that can be foreseen for the years 2005-2010 both from thestandpoint of the sulfur content and the standpoint of the content ofaromatic compounds and the value of the desired octane number. Thus, itis possible to cite, for example, the process that is described in U.S.Pat. No. 4,131,537 that teaches that it is advantageous to fractionatethe gasoline into several fractions, preferably three, as a function oftheir boiling point, and to desulfurize them under conditions that maybe different and in the presence of a catalyst that comprises at leastone metal of group VIB and/or of group VIII. This patent indicates thatthe greatest benefit is obtained when the gasoline is fractionated intothree fractions and when the fraction that has intermediate boilingpoints is treated under mild conditions.

[0007] It is also possible to cite the teaching of Patent ApplicationEP-A-0 725 126 that describes a process for hydrodesulfurization of acracking gasoline in which the gasoline is separated into a number offractions comprising at least a first fraction that is rich in compoundsthat are easy to desulfurize and a second fraction that is rich incompounds that are difficult to desulfurize. Before carrying out thisseparation, it is necessary to determine in advance the distribution ofthe sulfur-containing compounds by means of analyses. These analyses arenecessary for selecting the equipment and the separation conditions.

[0008] In this application, it is thus indicated that the olefin contentand the octane number of a light cracking gasoline fraction dropsignificantly when the fraction is desulfurized without beingfractionated. In contrast, the fractionation of said light fraction into7 to 20 fractions, followed by analyses of the sulfur and olefincontents of these fractions, makes it possible to determine the fractionor fractions that are the richest in sulfur-containing compounds, whichare then desulfurized simultaneously or separately and mixed with otherfractions that may or may not be desulfurized. Such a procedure iscomplex and should be implemented at each change in the composition ofthe gasoline that is to be treated.

[0009] Processes for hydrotreatment of gasolines consisting infractionating the gasoline, then in introducing the fractions atdifferent levels of a hydrodesulfurization reactor and in converting thedesulfurized fractions on a ZSM-5 zeolite to compensate for the octaneloss that is recorded by means of an isomerization, have also beenproposed in, for example, U.S. Pat. No. 5,290,427. This isomerization isaccompanied by a cracking of the gasoline into lighter compounds.

[0010] In these processes, the gasolines that are to be treated ingeneral have a starting point that is greater than 70° C., and thereagain it is necessary to treat the light gasoline (fraction thatcorresponds to the compounds with boiling points between C5 hydrocarbonswith 5 carbon atoms and 70° C.) separately, for example by softening.

[0011] According to the teaching of U.S. Pat. No. 5,599,441, the processfor desulfurization of a naphtha fraction comprising an olefinicgasoline comprises a stage for increasing the weight ofsulfur-containing compounds by alkylation followed by a stage forfractionation of the gasoline into two fractions, whereby the lightgasoline becomes depleted of sulfur.

[0012] According to the teaching of U.S. Pat. No. 6,024,865, the processfor the production of desulfurized gasoline comprises a separation ofthe gasoline into a light gasoline and a heavy gasoline. The lightgasoline is sent to a reactor for increasing the weight ofsulfur-containing compounds by alkylation. The heavy gasoline that issupplemented with reactive olefins is sent to a reactor for increasingthe weight of sulfur-containing compounds by alkylation. The heavygasoline and the light gasoline are mixed then sent to a distillationthat makes it possible to recover, at the top, a gasoline that isdepleted of sulfur.

[0013] According to the teaching of the document of Patent EP-A-1 077247 of the applicant, the gasoline that is to be desulfurized is treatedin a process that comprises a stage a1) for selective hydrogenation ofdiolefinic compounds, optionally at least one stage a2) that aims atincreasing the molecular weight of the light sulfur-containing productspresent in the starting gasoline to be desulfurized, at least one stageb) for separation of the gasoline that is obtained at the outlet ofstage a1) and/or a2) into two separate fractions including a lightfraction that is virtually lacking in sulfur and that contains thelightest olefins of the starting gasoline and the other heavy fractionthat contains almost all of the sulfur-containing compounds that arepresent in the starting gasoline and/or sulfur-containing compounds thatoriginate from stage a2), at least one stage c) for treatment of theheavy fraction that makes it possible to decompose or to hydrogenate theunsaturated sulfur-containing compounds that are present in thisfraction and in particular the cyclic, and even aromaticsulfur-containing compounds under limited hydrogenation conditions ofthe olefins that are present in this fraction and at least one stage d)of the gasoline that originates from stage c) that makes it possible todecompose the unsaturated sulfur-containing compounds that are nottransformed in stage c), and the linear and/or cyclic saturatedsulfur-containing compounds that are present in this gasoline thatoriginates from stage c) with a limited hydrogenation of the olefinsthat are present. The application of this process brings about alimitation of the temperature at the top of the fractionation column tobe able to obtain a top fraction that contains virtually no sulfur andin particular does not contain thiophene. This limitation is thus forthe most part detrimental as regards the use of the entire process,since the fraction that can be recovered at the top of the fractionationcolumn is limited in temperature and therefore in amount, which furtherbrings about an increase of the heavy fraction, which is then to betreated again with a view to reducing its sulfur content.

[0014] This invention makes it possible to produce a desulfurizedgasoline while limiting the octane loss by desulfurization. The schemeof this invention is distinguished from standard desulfurization schemesthat are based on a desulfurization of the total gasoline. In somecases, these schemes provide a separation of the gasoline into at leasttwo fractions: one fraction that contains the light gasoline and afraction that contains the heavy fraction, i.e., a gasoline with a finalboiling point that is higher than the one of the light gasoline.

[0015] The heavy gasoline is desulfurized by, for example, treatmentwith hydrogen or by absorption of sulfur-containing compounds. Theschemes that comprise a hydrogen treatment lead to a significantsaturation of olefins, and consequently to a high octane loss.

[0016] The light gasoline can be desulfurized under mild conditions byhydrodesulfurization, washing by a basic solution or adsorption onsolid, or softened by an oxidation process such as, for example, theMerox® process of the Universal Oil Product (UOP) Company which can beextractive.

[0017] Some schemes make it possible to desulfurize only the heavygasoline, the sulfur-containing compounds that are usually present inthe light gasoline (mercaptans) whose weight has been increased inadvance upstream from a distillation zone or fractionation zone(splitter in English).

[0018] As far as the processes for increasing the weight ofsulfur-containing compounds by alkylation presented here are concerned,they make it possible to produce a desulfurized light gasoline, wherebythe heavy gasoline concentrates the heavy sulfur-containing compoundsand the sulfur-containing compounds that are increased in weight byalkylation. These processes therefore do not make it possible todesulfurize the entire conversion gasoline fraction and particularly theone that is obtained from catalytic cracking units. Furthermore, theweight-increasing processes of the sulfur-containing compounds byalkylation are based on an acid catalysis. The service life of thecatalyst can be limited by the presence of diolefins and basic compounds(primarily nitrogen-containing compounds) in the gasoline that is to betreated.

SUMMARY OF THE INVENTION

[0019] This invention relates to a process for the production ofgasolines with low sulfur contents, which makes it possible to upgradethe entire gasoline fraction that contains sulfur, preferably a gasolinefraction of catalytic cracking or coking (according to Englishterminology), or pyrolysis, or else visbreaking (according to Englishterminology), optionally mixed with a direct distillation gasoline, andto reduce the sulfur contents in said gasoline fraction to very lowlevels, without appreciable reduction of the gasoline yield whileminimizing the reduction of the octane number that is caused by thehydrogenation of the olefins. The feedstock of the process according tothe invention can also optionally comprise in addition a gasolinefraction, whereby a C4 fraction comprises hydrocarbons with two, threeor four carbon atoms.

[0020] This invention relates to a process for the production ofgasolines with low sulfur contents from a starting gasoline thatcontains at least 150 parts per million by weight (ppm), often at least200 ppm and most often at least 300 ppm of sulfur-containing compoundscomprising at least the following stages:

[0021] a stage a) for selective hydrogenation of non-aromatic,polyunsaturated compounds that are present in the starting gasoline,

[0022] at least one stage b) that aims at increasing the molecularweight of the light sulfur-containing products, primarily those that arein the form of mercaptans that have 1 to 6 carbon atoms in theirmolecules and sulfides that often have 2 to 6 carbon atoms in theirmolecules that are initially present in the gasoline that is introducedin stage a) and/or those that are contained in the product thatoriginates from stage a),

[0023] at least one stage c) for alkylation of at least a portion of thesulfur-containing compounds, primarily those that are in the form ofthiophenic compounds that are present in the product that originatesfrom stage b) that aims at obtaining sulfur-containing compounds with ahigher molecular weight,

[0024] at least one stage d) for fractionation of the gasoline thatoriginates from stage c) into at least two fractions, a first fractionthat is virtually lacking in sulfur and that contains the lightestolefins of the unconverted starting gasoline in stage c), (lightgasoline), at least one other fraction, heavier than said firstfraction, enriched with sulfur-containing compounds and preferably atleast one so-called heavy fraction that contains the majority of thesulfur-containing compounds that are present in the gasoline thatoriginates from stage c) (heavy gasoline), and

[0025] at least one stage e) for treatment of at least one of theheavier fractions that originates from stage d) on a catalyst that makesit possible to decompose at least partially the sulfur-containingcompounds. Preferably during this stage e), the so-called heavy fractionthat is separated at stage d) is treated.

[0026] In stages b) and c), the term “primarily” means that during thestage in question, at least 50% of the sulfur-containing compounds thatare in the form of the compounds of the indicated type are converted.

[0027] Often, stage e) is a stage for treatment of at least one of theheaviest fractions that is separated in stage d) on a catalyst thatmakes it possible to decompose at least partially the sulfur-containingcompounds carried out under conditions where the hydrogenation of theolefins on this catalyst is limited.

[0028] According to a preferred embodiment of the invention, thetreatment of at least one of the heavier fractions that originates fromstage d) and preferably the heavy gasoline is carried out in two stages:

[0029] at least one stage e) for treatment of at least one of theheavier fractions and preferably the heavy gasoline that is separated instage d) on a catalyst that makes it possible to decompose at leastpartially the sulfur-containing compounds, under conditions where thehydrogenation of olefins on this catalyst is limited, and

[0030] at least one stage f) for treatment of the product that isobtained in stage e), without elimination of the H₂S that is formedduring this stage e), on a catalyst and under conditions that make itpossible to decompose at least partially the sulfur-containing compoundsthat are not transformed during stage e) with a limited hydrogenation ofolefins, and preferably to decompose the unsaturated sulfur-containingcompounds and the linear and/or cyclic saturated, sulfur-containingcompounds that are not transformed during stage e) with a limitedhydrogenation of olefins.

[0031] Within the meaning of this description, the term polyunsaturatedcompounds covers the diolefinic hydrocarbon-containing compounds, i.e.,that contain two double bonds, whereby the hydrocarbon-containingcompounds contain more than two double bonds but are not aromatic, andthe hydrocarbon-containing compounds contain at least one triple bondand are present in the starting gasoline. Most often, the startinggasoline contains essentially diolefinic compounds as polyunsaturatedcompounds. Within the meaning of this description, the gasoline with alow sulfur content that is obtained is a gasoline that contains lessthan 150 ppm by weight, often less than 100 ppm and most often less than80 ppm by weight of sulfur.

[0032] Stage b) for transformation of light sulfur-containing compoundsusually essentially relates to the transformation of saturatedsulfur-containing compounds that have a boiling point that is less than120° C. This stage can optionally be carried out simultaneously to stagea) over all or part of the starting gasoline, in the same reactor or ina different reactor. It can also be carried out separately over all orpart of the hydrogenated gasoline in stage a). In this stage b), thethiophene and the thiophenic compounds undergo little transformation.This stage a) for hydrogenation of unsaturated compounds and stage b)that aims at increasing the molecular weight of the saturated lightsulfur-containing products that are initially present in the gasolinethat is introduced in stage a) thus are optionally carried outsimultaneously in a single reaction zone that contains one or more bedsof a single catalyst.

[0033] Stage c) is a stage for alkylation of the sulfur-containingcompounds that usually belong to the group that comprises the thiophene,the thiophenic compounds and the mercaptans that are present in theproduct that originates from stage b). The optionally present mercaptansare either those that are formed in stage a) and/or b) or those that arepresent in the starting gasoline and are not converted in stage a)and/or b). This stage can be carried out in one or more reactors inseries or in parallel. In the case of a device that comprises severalreactors in series, the first reactor may act as a guard bed that isintended to hold up the basic compounds that are optionally present inthe starting gasoline. These basic compounds can also be at least inpart eliminated before the introduction of gasoline into stage a) forhydrogenation of the polyunsaturated compounds. This elimination ispreferably carried out by a treatment with an acid aqueous solution. Theoperating conditions can be adjusted such that a portion of the startinggasoline olefins are converted into long olefins that are branched byaddition reactions between olefins. The operating conditions also can beadjusted so that a portion of the aromatic compounds is increased inweight by alkylation by olefins.

[0034] Stage d) for fractionation of the gasoline that is obtained atthe end of stage c) comprises the fractionation into at least twofractions: a light fraction that is preferably virtually lacking insulfur and that contains the lightest olefins of the starting gasoline(light gasoline or light fraction), and a heavy fraction in whichpreferably the majority of the sulfur-containing compounds that areinitially present in the starting gasoline and/or formed during stagesa), b) and c) are concentrated (gasoline or heavy fraction). It is alsopossible to separate the gasoline that is obtained in stage c) into morethan two fractions, i.e., for example, a light fraction, at least oneintermediate fraction and a heavy fraction. In this case, theintermediate fraction or fractions can be sent to a catalytic reformingunit usually after an intensive desulfurization stage.

[0035] Stage e) for treatment of the heavy gasoline and/or anintermediate fraction that contains a proportion of sulfur-containingcompounds that is very considerably larger than the one contained in thelight fraction is a stage for treatment of this fraction on a catalystthat makes it possible to decompose at least partially thesulfur-containing compounds, in particular the cyclic sulfur-containingcompounds, and even aromatic compounds such as, for example, thethiophenic compounds, by being placed under conditions where thehydrogenation of the olefins on this catalyst is limited.

[0036] Stage f) is a stage for treatment of the product that is obtainedin stage e), without eliminating the H2S that is formed during thisstage e), on a catalyst and under conditions that make it possible todecompose or to hydrogenate at least partially the sulfur-containingcompounds that are not transformed during stage e), and preferably todecompose or to hydrogenate the unsaturated sulfur-containing compoundsand the linear and/or cyclic saturated sulfur-containing compounds thatare not transformed during stage e) and in particular the thiopheniccompounds and the mercaptans, with a limited hydrogenation of olefins.

[0037] Stages e) and f) are most often carried out in at least twosuccessive and separate reaction zones. The catalytic treatments thatare carried out during stages e) and f) can be carried out either in asingle reactor that contains the two catalysts, or in at least twodifferent reactors. When the treatment is carried out with two reactors,the latter two are placed in series, whereby the second reactorpreferably treats the effluent as a whole at the outlet of the firstreactor, preferably without separating the liquid and the gas betweenthe first and the second reactor. It is also possible to use severalreactors in parallel or in series for one and/or the other of stages e)and/or f).

[0038] Furthermore, it is not necessary to eliminate the H2S that isformed during stage e) before sending the effluent from this stage e) tothe inlet of the hydrogenation reactor or reactors of stage f).

[0039] One of the advantages of the process according to the inventiontherefore resides in the fact that it is not necessary to adjust the H2Scontent between stage e) and stage f).

[0040] Furthermore, a stage g) is preferably carried out after stage f),and this stage consists in mixing the light gasoline that is separatedin stage d) and at least a portion of the heavy gasoline that originatesfrom stage f) to form the overall desulfurized gasoline that is desired.

[0041] The entire desulfurized heavy gasoline that originates from stagef) is preferably mixed with the light gasoline that originates fromstage d), without separation of the liquid and gas contained in theheavy gasoline after desulfurization; optionally a simple stripping byat least one cover gas can be carried out to eliminate the H2S of theextensively desulfurized heavy gasoline, i.e., that usually containsless than 50% by weight and often less than 20% by weight of residualsulfur-containing compounds relative to the content of sulfur-containingcompounds of the heavy gasoline that exits stage d).

[0042] In some specific cases, the upgrading of the light gasoline andthe desulfurized heavy gasoline is carried out separately. It is thenunnecessary to carry out stage g).

[0043] For the gasolines whose final boiling point is greater than 130°C., two additional stages may be necessary:

[0044] an additional stage for elimination of basic compounds that canbe carried out by washing with an acid aqueous solution that is intendedto eliminate the basic compounds may be necessary before stage c)

[0045] a stage for fractionation of the gasoline that originates fromstage d) for fractionation or else one of stages e) or f) forhydrodesulfurization that is intended to separate from the gasolineproduced a heavy fraction whose starting point would be, for example,210° C., and a lighter fraction whose end point would be, for example,210° C.

[0046] In the case where this fractionation is carried out on thegasoline that originates from stage d), the light fraction is sent intostage e).

[0047] The feedstock of the process according to the invention is agasoline fraction that contains sulfur, preferably a gasoline fractionthat originates from a cracking unit, most often from a catalyticcracking unit, whose range of boiling points typically extends fromapproximately the boiling points of hydrocarbons with 2 or 3 carbonatoms (C2 or C3) up to about 250° C., preferably from approximately theboiling points of hydrocarbons with 2 or 3 carbon atoms (C2 or C3) up toabout 220° C., more preferably from about the boiling points ofhydrocarbons with 5 carbon atoms (C5) up to about 220° C. The process ofthis invention applies more particularly to the catalytic crackinggasolines whose final boiling points are from about 120° C. to about230° C.

[0048] The advantages of this scheme relative to those of the prior artare as follows:

[0049] 1. Removal of diene compounds and optionally acetylene compounds,contained in the starting gasoline with a view to limiting thedeactivation of the alkylation catalyst of subsequent stage c) of thesulfur-containing compounds as well as the formation of gums in thegasolines.

[0050] 2. Elimination of light mercaptans for producing a softened lightgasoline that does not require purification or subsequentdesulfurization.

[0051] 3. Increase of the production of desulfurized light gasoline thatis recovered at the top of the fractionation column, which induces areduction of the amount of gasoline to be treated byhydrodesulfurization.

[0052] 4. Production, by fractionation of a branched olefin-rich heavygasoline whose octane is not very sensitive to the saturation ofolefins.

[0053] 5. Selective desulfurization of the heavy gasoline (or result oflateral drawing-off) to limit the saturation of olefins and the octaneloss of heavy gasoline.

[0054] 6. Overall reduction of the vapor pressure of the gasoline thatis produced relative to the starting gasoline.

DETAILED DESCRIPTION OF THE INVENTION

[0055] This invention describes a process that makes it possible toobtain a desulfurized gasoline from a conversion gasoline and preferablyfrom a gasoline that originates from a unit for catalytic cracking,coking, visbreaking, or pyrolysis optionally mixed with a saturatedgasoline and that has a limited sulfur content in which the gasolinefirst undergoes a selective hydrogenation treatment of diolefins, then astage for transformation of sulfur-containing compounds of the gasolinethat are present in the form of mercaptans that have 1 to 6 carbon atomsin their molecule and sulfides that have often 2 to 6 carbon atoms,which, after fractionation, should be found in the light gasoline suchthat they are essentially in the heavy fraction after the fractionationstage of the process according to the invention, a stage for alkylationof the sulfur-containing compounds that are present in the form ofthiophenic compounds in the gasoline that originates from the stage fortransformation of the sulfur-containing compounds in the form ofmercaptans and sulfides. In this stage, the operating conditions can beadjusted so as to promote the alkylation reactions of olefins on olefinsand aromatic compounds, which causes, in particular, a reduction in thevapor pressure of the gasoline. At least one fraction that originatesfrom the fractionation stage, preferably the heavy fraction or anintermediate fraction, can be treated in a stage for transformation ofthese sulfur-containing compounds into H2S (hydrogen sulfide) with orwithout reduction of the saturation of olefins.

[0056] After said fractionation, the heavy gasoline or at least oneintermediate fraction is treated in a hydrodesulfurization section,preferably in the presence of a hydrodesulfurization catalyst oroptionally an absorbent. Preferably no desulfurization of the lightfraction is necessary in the process according to the invention, sincethe bulk of the initially present sulfur-containing compounds in thegasoline are in the heavy fraction and optionally in the intermediatefraction or fractions of fractionation stage d) that is carried outafter the stages for hydrogenation, for transformation of thesulfur-containing compounds (stage b), for transformation of thiopheniccompounds and optionally mercaptans, in particular residual mercaptansthat are not converted and/or formed in stages a) and b) (stage c) byalkylation of at least a portion of the unconverted sulfur-containingcompounds in stage b) (stage c).

[0057] This scheme makes it possible to obtain in fine a desulfurizedgasoline that has no significant reduction in the olefin content or theoctane number even for high desulfurization rates; and this is sowithout it being necessary to treat the light gasoline by means of ahydrodesulfurization section or softening section, or to have recourseto processes that make it possible to recover the octane number of thegasoline. Thanks to this process, significant desulfurization rates areachieved under reasonable operating conditions that are specified below.

[0058] The sulfur-containing radicals that are contained in thefeedstocks that are treated by the process of the invention can bemercaptans, sulfides, disulfides and/or heterocyclic compounds, such as,for example, thiophene or alkyl-thiophenes, or heavier compounds, suchas, for example, benzothiophene and/or dibenzothiophene.

[0059] The fractionation point of the gasoline is preferably limited soas to prevent the presence of sulfur-containing compounds in the lightgasoline. In the absence of the reactor for alkylation ofsulfur-containing compounds, it would be possible to separate in thelight gasoline only the C5 olefins and a small portion of the C6 olefinsfor fear of entraining too large a thiophene fraction into thisfraction. Thus, the process according to the invention is advantageousby carrying out a stage for transformation of the thiophene and moregenerally thiophenic compounds, for example with an alkylation sectionupstream from the fractionation section or integrated in said sectionaccording to a detailed embodiment in the description below.

[0060] To make it possible to recover a larger fraction of the lightgasoline while limiting the sulfur content of this fraction withoutadditional treatment, it is preferably proposed to treat the feedstockin stage b) under conditions and on catalysts that make it possible totransform the light sulfur-containing compounds that are present in theform of mercaptans and sulfides that most often have 2 to 6 carbon atomsin their molecule into sulfur-containing compounds with a higher boilingpoint that is found after the separation, optionally in at least oneintermediate fraction or in the heavy gasoline. These intermediateand/or heavy fractions can then be desulfurized. This desulfurization iscarried out under definite conditions and by means of ahydrodesulfurization catalyst that optionally makes it possible to limitthe saturation of the olefins or, according to a preferred embodiment ofthe invention, by means of a scheme of catalysts that makes it possibleto reach high desulfurization rates while limiting the hydrogenationrate of the olefins and therefore the octane loss.

[0061] The sulfur content of the gasoline fractions that are produced bycatalytic cracking and in particular fluidized-bed catalytic cracking(FCC) depends on the sulfur content of the feedstock that is treatedwith FCC., the presence or absence of a pretreatment of the feedstock ofthe FCC., as well as the end point of the fraction. Generally, thesulfur contents of the entire gasoline fraction, in particular the onesthat are obtained from the FCC., are greater than 150 ppm by weight andmost often greater than 500 ppm by weight. For gasolines that have endpoints that are greater than 200° C., the sulfur contents are oftengreater than 1,000 ppm by weight, and they can even in some cases reachvalues on the order of 4,000 to 5,000 ppm by weight.

[0062] The process according to the invention applies particularly whenhigh desulfurization rates of the gasoline are required, i.e., when thedesulfurized gasoline should contain at most 10% of the sulfur of thestarting gasoline and optionally at most 5% and even at most 2% of thesulfur of the starting gasoline that corresponds to desulfurizationrates that are greater than 90% and even greater than 95% or 98%.

[0063] The process according to the invention comprises at least stagesa) to e) below:

[0064] Stage a) for selective hydrogenation of diolefins: this stage isintended to eliminate the diolefins that can bring about a prematuredeactivation of the catalysts for increasing the weight ofsulfur-containing compounds by alkylation during stage c) below and toform gums in the desulfurized gasolines. This stage is carried out bypassage of the effluent, preferably consisting of the entire gasolinefraction, on a catalyst that makes it possible to hydrogenateselectively the diolefins of the gasoline without hydrogenating theolefins.

[0065] Stage b) for increasing the weight of mercaptans and lightsulfides on the supported metal-based catalyst: this reaction can beimplemented in the reactor for selective hydrogenation of diolefins.This stage consists in running all or part of the starting gasoline orthe gasoline that is hydrogenated in stage a), preferably all of thestarting gasoline or gasoline that is hydrogenated in stage a), over acatalyst that makes it possible to transform at least in part the lightsulfur-containing compounds (for example: ethylmercaptan, propylmercaptan), by reaction with all or part of the olefins, into heaviersulfur-containing compounds. This stage is carried out, for example,simultaneously to stage a) by running, for example, the startinggasoline over a catalyst that can both hydrogenate the diolefins andtransform the light sulfur-containing compounds, preferably witholefins, into-heavier sulfur-containing compounds, or over a separatecatalyst but that makes it possible to carry out this transformation inthe same reactor as the one that is used for carrying out stage a).

[0066] Stage c) is a stage for increasing the weight ofsulfur-containing compounds by alkylation. This stage makes it possibleto eliminate the majority of the sulfur-containing compounds that arepresent in the form of thiophenic compounds by a reaction of addition tothe olefins. This stage consists in running all or part of the gasolinethat is obtained from stage b) over a catalyst that has an acid functionthat makes it possible to carry out the addition of sulfur-containingcompounds in the form of mercaptans and sulfides to the olefins and thealkylation reaction of the thiophene and thiophenic derivatives by thesesame olefins.

[0067] Stage d) for fractionation of the gasoline that is obtained atthe end of stage c): this stage is intended to produce a lightdesulfurized gasoline at the top of the distillation column. Thegasoline that is thus recovered is low in sulfur and most often does notrequire additional treatment. This light gasoline that is low in sulfuris most often sent directly into the storage zone of the gasoline(gasoline pool according to the terminology used most frequently in theart), preferably without additional post-treatment. The gasoline that isrecovered at the bottom of the column concentrates the sulfur-containingcompounds that are initially present in the feedstock. This column cancomprise lateral drawing-off making it possible to obtain, for example,one or more intermediate fractions.

[0068] In stage e), the gasoline at the bottom of the column and/or theone that is contained in an intermediate fraction that originates fromstage d) is desulfurized by hydrotreatment.

[0069] According to a preferred embodiment of the invention, thegasoline at the bottom of the column and/or the one that is contained inan intermediate fraction that originates from stage d) is desulfurizedin stage e) on a catalyst and under conditions such that the olefinsaturation is partial to limit the octane loss.

[0070] According to a more preferred embodiment of the invention, insuccessive stages e) and f), the gasoline at the bottom of the columnand/or the one that is contained in an intermediate fraction thatoriginates from stage d) is desulfurized by hydrotreatment according toa process that makes it possible to limit the saturation of the olefins.Actually, the implementation of a selective hydrodesulfurization makesit possible to limit the saturation of olefins and thus to limit theloss of octane of the gasoline. The heavy gasoline and/or at least oneintermediate gasoline that is thus desulfurized can then optionally bestripped (i.e., a gas stream that preferably contains one or more covergases is run through this gasoline) so as to eliminate the H₂S that isoptionally produced during the desulfurization.

[0071] According to a variant of the process according to the invention,it is possible to associate at least one reaction section with thefractionation column. Said reaction section or sections then operate onat least one fraction that is sampled inside the fractionation column,and the effluent of the reaction section is sent to the fractionationcolumn. The reaction section or sections thus coupled to thefractionation column of stage d) can be selected from the group thatconsists of the reaction sections of the following stages:

[0072] transformation of sulfur-containing compounds by alkylation, suchas, for example, thiophene, thiophenic compounds and optionallymercaptans (stage c)

[0073] desulfurization of intermediate fractions and/or desulfurizationof the heavy fraction ((stage e) and/or stage f)).

[0074] Such devices that comprise a fractionation column combined withan external reactor that can be used in the process according to theinvention were described, for example, for applications in the field ofrefining and petrochemistry in U.S. Pat. Nos. 5,1777,283, 5,817,227 and5,888,355.

[0075] According to other variants of the process according to theinvention, it is also possible to use a reactive column instead of thefractionation column, i.e., to place at least one of said reactionsections in the fractionation column (reaction section that is inside ofthe column), preferably in a zone where the reagent concentration is ata maximum. Thus, for example, in the case of stage c) for transformationof sulfur-containing compounds by alkylation, such as, for example, thethiophenic compounds, the reaction section will preferably be placed ina zone that exhibits the maximum concentration of these compounds.

[0076] According to a preferred variant of the process according to theinvention, the reaction section that is inside of the column is selectedfrom the group that consists of the following reaction sections:transformation of sulfur-containing compounds such as thiophene,thiophenic compounds and optionally mercaptans by alkylation (stage c)),desulfurization of the intermediate fractions and/or desulfurization ofthe heavy fraction ((stage e) and/or stage f).

[0077] In a very preferred embodiment of the process according to theinvention, the reaction section is placed in the middle of afractionation column so as to treat the compounds that have intermediateboiling points, e.g., the compounds that can constitute an intermediatefraction and that are recovered alone or with the heavy fraction at thebottom of the column, at the end of the fractionation stage. The heavyfraction is then treated in an external reactor that may or may not becombined with the fractionation column.

[0078] Such reactive columns are known to one skilled in the art andhave been described in, for example, Patents or Patent Applications U.S.Pat. No. 5,368,691, U.S. Pat. No. 5,523,062, FR 2,737 131, FR 2,737,132,and EP-A-0 461 855.

[0079] Another variant of the process according to the inventionconsists both in using a reactive column that comprises at least onereaction section and an external reactor that may or may not be coupledto said column. Such variants are described in, for example, PatentApplication WO 00/15319.

[0080] The variants that are described above are only illustrations ofpossible variants of the process according to the invention. The processaccording to the invention actually can be implemented by combiningreaction sections (stages a), b), c), e) or f)) either associated withthe fractionation column of stage d), or inside of said column, oroutside and not coupled to said column in that the effluent of saidreaction section or sections is not recycled to the fractionationcolumn.

[0081] One of the advantages of the process according to the inventionresides in the fact that most often it is not necessary to desulfurizethe light fraction of the gasoline that originates from fractionation.The transformation of sulfur-containing compounds and/or thiopheniccompounds (stages b) and/or c)) actually makes it possible tosignificantly reduce the content of sulfur-containing compounds of thelight fraction and optionally at least one intermediate fraction, andgenerally to recover the bulk of these compounds in the heavy fractionand optionally in the intermediate fraction or fractions.

[0082] Stages b) and c) are distinguished from one another by the factthat the conversion of the thiophenic compounds is generally less than60% by weight, and even less than 40% by weight in stage b), while theconversion is most often greater than 80% by weight, preferably greaterthan 90% by weight, very preferably greater than 95% by weight in stagec). Stage b) actually essentially makes the mercaptans and lightsulfides heavier whereas stage c) essentially makes the thiopheniccompounds heavier.

[0083] This operation is carried out while maintaining the bulk of theolefins in the light fraction, optionally in at least one intermediatefraction that does not require intensive desulfurization. The content ofsulfur-containing compounds of the light fraction that is thus obtainedis generally less than 100 ppm, preferably less than 50 ppm, morepreferably less than 20 ppm and very preferably less than 10 ppm.

[0084] Another advantage resides in the fact that the residual contentof sulfur-containing compounds of the desulfurized gasoline by means ofthe process according to the invention is particularly low and that theoctane number of the gasoline is kept at a high level.

[0085] The stages of the process according to the invention aredescribed in more detail below.

[0086] Hydrogenation of Diolefins (Stage a):

[0087] The hydrogenation of dienes is a stage that makes it possible toeliminate, before hydrodesulfurization, almost all of the dienes thatare present in the gasoline fraction that contains the sulfur to betreated. It preferably takes place in the first stage (stage a) of theprocess according to the invention, generally in the presence of acatalyst that comprises at least one metal of group VIII, preferablyselected from the group that consists of platinum, palladium and nickel,and a substrate. For example, a nickel-based or palladium-based catalystthat is deposited on an inert substrate, such as, for example, alumina,silica or a substrate that contains at least 50% of alumina, will beused.

[0088] The pressure that is used is adequate for maintaining more than60%, preferably 80%, and more preferably 95% by weight of the gasolinethat is to be treated in liquid phase in the reactor; it is mostgenerally between about 0.4 and about 5 MPa and preferably greater than1 MPa, more preferably between 1 and 4 MPa. The hourly volumetric flowrate of the liquid that is to be treated is between about 1 and about 20h⁻¹ (volume of feedstock per volume of catalyst and per hour),preferably between 2 and 10 h⁻¹, very preferably between 3 and 8 h⁻¹.The temperature is most generally between about 50 and about 250° C.,and preferably between 80 and 220° C., and more preferably between 100and 200° C., to ensure an adequate conversion of diolefins. Verypreferably, it is limited to 180° C. The hydrogen to feedstock ratiothat is expressed in liters is generally between 1 and 50 liters perliter, preferably between 2 and 30 liters, more preferably between 3 and25 liters per liter.

[0089] The selection of operating conditions is particularly important.The operation most generally will be performed under pressure and in thepresence of an amount of hydrogen that slightly exceeds thestoichiometric value that is necessary for hydrogenating the diolefins.The hydrogen and the feedstock that is to be treated are injected inupward or downward flows in a reactor that preferably comprises a fixedcatalyst bed.

[0090] Another metal can be combined with the main metal to form abimetallic catalyst, such as, for example, molybdenum or tungsten. Theuse of such catalytic formulas has been claimed in, for example, PatentFR 2 764 299.

[0091] The catalytic cracking gasoline can contain up to several percent(%) by weight of diolefins. After hydrogenation, the diolefin content isgenerally reduced to less than 3000 ppm, and even less than 2500 ppm,and more preferably less than 1500 ppm. In some cases, less than 500 ppmcan be obtained. The diene content after selective hydrogenation caneven, if necessary, be reduced to less than 250 ppm.

[0092] According to a particular embodiment of the process according tothe invention, the hydrogenation stage of the dienes takes place in acatalytic hydrogenation reactor that comprises a catalytic reaction zonethat is traversed by all of the feedstock and the amount of hydrogenthat is necessary to carry out the desired reactions.

[0093] Transformation of Light Sulfur Compounds (Stage b):

[0094] This stage consists in transforming the light sulfur compounds,i.e., the compounds that would be found at the end of stage a) forhydrogenation of dienes (after fractionation in stage d)) in lightgasoline, into heavier sulfur-containing compounds that are entrained inthe heavy gasoline. The light sulfur-containing compounds preferably areselected from the families of mercaptans with 1 to 6 carbon atoms andsulfides with 2 to 6 carbon atoms. This transformation is preferablycarried out on a catalyst that comprises at least one element of groupVIII of the periodic table (Handbook of Chemistry and Physics 45thEdition 1964-1965) (groups 8, 9 and 10 of the new periodic table). Theselection of the catalyst is carried out in particular to promote thereaction between the light mercaptans and the olefins, which producesheavier sulfides or mercaptans. Other compounds such as COS or CS2 canoptionally also be converted.

[0095] This stage optionally can be carried out at the same time asstage a). For example, it can be particularly advantageous to operate,during the hydrogenation of the diolefins, under conditions such that atleast a portion of the compounds in mercaptan form are transformed. Acertain reduction of the mercaptan content thus is obtained. To do this,it is possible to use the procedure for hydrogenating dienes that isdescribed in Patent Application EP-A-0 832 958, which advantageouslyuses a palladium-based catalyst, or the one that is described in PatentFR 2 720 754.

[0096] Another possibility is to use a nickel-based catalyst that isidentical to or different from the catalyst of stage a), such as, forexample, the catalyst that is recommended in the process of U.S. Pat.No. 3,691,066, which makes it possible to transform the mercaptans(butylmercaptan) into heavier sulfur-containing compounds (sulfides).

[0097] Another possibility for carrying out this stage consists inhydrogenating at least partly the thiophene into thiophane whose boilingpoint is greater than that of thiophene (boiling point 121° C). Thisstage can be carried out on a catalyst with a nickel, platinum orpalladium base. In this case, the temperatures are generally between 100and 300° C., and preferably between 150 and 250° C. The H2/feedstockratio is adjusted between 1 and 20 liters per liter, preferably between3 and 15 liters per liter, to promote, if possible, in addition thedesired hydrogenation of the thiophenic compounds and to reduce thehydrogenation of the olefins that are present in the feedstock. Thevolumetric flow rate is generally between 1 and 10 h⁻¹, preferablybetween 2 and 4 h⁻¹, and the pressure is between 0.5 and 5 MPa,preferably between 1 and 3 MPa.

[0098] Transformation of Sulfur-Containing Compounds by Alkylation(Stage c):

[0099] This stage consists in preferably running the entire fractionthat originates from stage b) over a catalyst that has an acid functionthat makes it possible to carry out the addition of sulfur-containingcompounds in the form of mercaptans over olefins and the alkylationreaction of thiophene and thiophenic derivatives by these same olefins.The operating conditions are adjusted to carry out the desiredtransformation with conversion rates of thiophene and/or thiopheniccompounds that are greater than 80% by weight, preferably greater than90% by weight, very preferably greater than 95% by weight. Othercompounds such as COS or CS2 can optionally also be converted.

[0100] During this stage, a portion of the olefins of the startinggasoline can be converted into branched long olefins by additionreactions (oligomerization) between olefins, and a portion of thearomatic compounds increased in weight by alkylation by the olefins.

[0101] To reduce the oligomerizing activity of the acid catalyst that isoptionally used, the gasoline can be supplemented with a known compoundfor inhibiting the oligomerizing activity of the acid catalysts, such asthe alcohols, ethers or water.

[0102] During the alkylation, the thiophenic compounds whose boilingpoint is from about 60° C. to about 160° C. will react with conversionrates that are greater than 80% by weight, preferably greater than 90%by weight, with olefins to form thiophene alkyls whose boiling point isconsiderably larger than the one of the starting thiophenic compounds.

[0103] A portion or all of the benzene can also be eliminated byalkylation with the olefins.

[0104] These compounds of higher molecular weight are primarilycharacterized by higher boiling points than those that they had beforealkylation. Thus, the theoretical boiling point of the thiophene, whichis 84° C., is shifted toward 150° C. for the thiophene alkyls. Thisreaction is coupled to olefin addition reactions that most oftentherefore result in increasing the weight of the gasoline, in particularin the case where the gasoline fraction and/or the starting gasoline arelight, and to a reduction of its vapor pressure.

[0105] This alkylation stage is carried out in the presence of acidcatalyst. This catalyst can be equally a resin, a zeolite, a clay, anysilica that is functionalized or any silico-aluminate that has anacidity, or any grafted substrate of acid functional groups. The ratioof the injected feedstock volume to the catalyst volume is between 0.1and 10 liters/liter/hour and preferably between 0.5 and 4liters/liter/hour. More specifically, this alkylation stage is carriedout in the presence of at least one acid catalyst that is selected fromthe group that consists of Bronsted acids (including phosphoric acids,sulfuric acids, boric acids, hydrofluoric acids), supported on, forexample, silica, alumina or alumina silica, silicoaluminates,titanosilicates, mixed alumina-titanium compounds, clays, resins, mixedoxides obtained by grafting at least one organometallic compound that isorganosoluble or water-soluble (selected from the group that consists ofalkyls and/or alkoxy metals of at least one element such as titanium,zirconium, silicon, germanium, tin, tantalum, niobium...) on at leastone oxide such as alumina (gamma, delta, eta forms, alone or mixed),silica, alumina silicas, titanium silicas, zirconium silicas or anyother solid that has any acidity. A particular embodiment of theinvention can consist in using a physical mixture of at least two of thecatalysts above in proportions that vary from 95/5 to 5/95, preferablyfrom 85/15 to 15/85 and very preferably from 70/30 to 30/70.

[0106] The temperature for this stage is generally from about 10 toabout 350° C. according to the type of catalyst or the force of theacidity. Thus, for a supported phosphoric acid-type catalyst, thetemperature is usually from about 50 to about 250° C., preferably fromabout 100 to about 210° C.

[0107] The molar ratio of olefins to thiophenic compounds is greaterthan 10 mol/mol, preferably greater than 100 mol/mol.

[0108] The operating pressure of this stage is generally between 0.1 and3 MPa and preferably such that the feedstock is in liquid form under thetemperature and pressure conditions, or at a pressure that is higherthan 0.5 MPa.

[0109] At least part and preferably all of the effluent that originatesfrom stage c) for transformation of sulfur-containing compounds is sentinto a fractionation unit (stage d)) to be separated into at least twofractions: a light fraction and a heavy fraction that is preferably sentas a whole optionally after mixing with an intermediate fraction into adesulfurization zone that operates in one (stage e) or two stages forsuccessive desulfurization (stage e) and stage f)).

[0110] Separation of the Gasoline that Originates from Stage c) into atLeast Two Fractions (Stage d):

[0111] According to a first variant of the process according to theinvention, the gasoline is fractionated into two fractions:

[0112] a light fraction that contains a limited residual sulfur content,preferably less than about 100 ppm, very preferably less than about 20ppm, and that makes it possible most often to use this fraction withoutcarrying out other treatment that aims at reducing its sulfur content,

[0113] a heavy fraction in which the bulk of the sulfur that isinitially present in the feedstock is preferably concentrated.

[0114] This separation is preferably carried out by means of a standarddistillation column that is also called a splitter according to theEnglish name. This fractionation column should make it possible toseparate a light fraction of the gasoline that contains a small fractionof sulfur and a heavy fraction that preferably contains the bulk of thesulfur that was initially present in the starting gasoline.

[0115] This column generally operates at a pressure of between 0.1 and 2MPa and preferably between 0.2 and 1 MPa. The number of theoreticalplateaus of this separation column is generally between 10 and 100 andpreferably between 20 and 60. The reflux rate, which is expressed asbeing the ratio of the liquid flow rate in the column that is divided bythe distillate flow rate, expressed in kg/h, is generally less than theunit and preferably less than 0.8.

[0116] The light gasoline that is obtained at the end of the separationgenerally consists of hydrocarbon-containing fractions with 5, 6 and 7carbons. Generally, this light fraction has a low sulfur content, i.e.,it generally is not necessary to treat the light fraction before usingit as a fuel.

[0117] In this case, the gasoline is preferably fractionated into atleast two fractions that have the following properties:

[0118] a so-called light fraction (fraction L) whose boiling points arepreferably less than about 120° C. This temperature is given by way ofexample; it generally corresponds to the maximum temperature for whichthe sulfur content is less than 20 ppm,

[0119] at least one so-called heavy fraction (fraction H1), whoseboiling points are greater than about 100° C.

[0120] Light fraction L is preferably injected into a flask forgas-liquid separation so as to separate the unconsumed hydrogen and-theH2S, formed during stage a) and/or b) and/or c), whereby olefinsgenerally have 5 to 7 carbon atoms.

[0121] So-called heavy fraction H1, i.e., the fraction whosetemperatures are greater than about 100° C., is sent into thedesulfurization zone of stage e) and preferably stages e) and f).

[0122] According to a second variant of the process according to theinvention, the gasoline is fractionated into at least three fractions: alight fraction, a heavy fraction, and at least one intermediatefraction.

[0123] The light fraction is identical to the one that is describedabove. Intermediate fraction I2 whose boiling points by way of exampleare at least 100° C. and at most about 140° C. and even about 160° C.This fraction can be treated in stage e) then optionally in stage f) ofthe process according to the invention. Heavy fraction H2 is then afraction whose boiling points are generally greater than about 160° C.or about 140° C. In this case, the unit of intermediate fraction plusheavy fraction is equivalent to heavy fraction H1 of the case where thefractionation is limited to two fractions.

[0124] Heavy fraction H2 whose boiling points are generally greater thanabout 160° C. or about 140° C. is sent into the desulfurization zone.

[0125] In another version of the process according to the invention, itis also possible to fractionate the product that originates from stagec) into at least three fractions: a light fraction (L), at least oneintermediate fraction (I2) and at least one heavy fraction (H2) thatexhibits the properties described above.

[0126] Intermediate fraction I2 whose boiling points are between about100° C. and about 120° C. or about 160° C. can be sent into atransformation unit of sulfur-containing compounds according to stage c)or recycled in this stage c).

[0127] After stage d), fraction or fractions I2 can again befractionated into an intermediate fraction I3 and a heavy fraction H3.Fraction H3 that is thus obtained can optionally be mixed with fractionH2, preferably before desulfurization, and fraction I3 can be sent intoa unit for transformation of sulfur-containing compounds according tostage c) or recycled in this stage c).

[0128] In the description below, the conditions given for stage e)include the one in which a single desulfurization stage is carried outand those of a preferred embodiment of the invention in which thehydrodesulfurization is carried out in two successive stages e) and f).

[0129] Decomposition of the Sulfur-Containing Compounds of the HeavyFractions and/or Intermediate Fractions that Originate from Stage d)(Stage e):

[0130] This stage, which applies to the heavy gasoline (heavy fractionsand/or intermediate fractions) obtained at the end of fractionationstage d), consists in hydrogenolyzing at least partially thesulfur-containing compounds for forming the H₂S. The fraction of thesulfur-containing compounds that are thus transformed is a function ofthe desired desulfurization rate.

[0131] This stage can be carried out, for example, by running the heavygasoline, in the presence of hydrogen, over a catalyst that comprises atleast one element of group VIII and/or at least one element of group VIBat least in part in sulfide form, at a temperature of between about 210°C. and about 350° C., preferably between 220° C. and 320° C., under apressure of generally between about 1 and about 4 MPa, preferablybetween 1.5 and 3 MPa. The volumetric flow rate of the liquid is betweenabout 0.5 and about 20 h⁻¹ (expressed by volume of liquid per volume ofcatalyst and per hour), preferably between 0.5 and 10 h-1, verypreferably between 1 and 8 h-1. The H₂/HC. ratio is between 100 to 600liters per liter and preferably between 200 and 500 liters per liter.

[0132] To carry out, at least in part, the hydrogenolysis of theunsaturated sulfur-containing compounds of the gasoline according to theprocess of the invention, generally at least one catalyst that comprisesat least one element of group VIII (metals of groups 8, 9 and 10 of thenew periodic table, i.e., iron, ruthenium, osmium, cobalt, rhodium,iridium, nickel, palladium or platinum) and/or at least one element ofgroup VIB (metals of group 6 of the new periodic table, i.e, chromium,molybdenum or tungsten) is used on a suitable substrate.

[0133] The metal content of group VIII that is expressed in oxide isgenerally between 0.5 and 15% by weight, preferably between 1 and 10% byweight. The metal content of group VIB is generally between 1.5 and 60%by weight, preferably between 3 and 50% by weight. The element of groupVIII, when it is present, is preferably cobalt, and the element of groupVIb, when it is present, is generally molybdenum or tungsten.Combinations such as cobalt-molybdenum are preferred. The substrate ofthe catalyst is usually a porous solid, such as, for example, analumina, a silica-alumina or other porous solids, such as, for example,magnesia, silica or titanium oxide, alone or mixed with alumina orsilica-alumina. To reduce the hydrogenation of the olefins that arepresent in the heavy gasoline, it is advantageous preferably to use acatalyst in which the molybdenum density, expressed in % by weight ofMoO3 per unit of surface area, is greater than 0.07 and preferablygreater than 0.10. The catalyst according to the invention preferablyhas a specific surface area that is less than 190 m2/g, more preferablyless than 180 m2/g, and very preferably less than 150 m2/g.

[0134] The catalyst is preferably used at least in part in itssulfurized form. The sulfurization stage can be carried out by anytechnique that is known to one skilled in the art, in situ or ex situ.

[0135] In the process according to the invention, the conversion of thesulfur-containing compounds is greater than 50% and preferably greaterthan 90%.

[0136] Stage e) is implemented under conditions such that thetransformation of at least a portion of the unsaturated compounds of thesulfur, such as the thiophenic compounds, into saturated compounds, forexample, into thiophanes (or thiacyclopentanes) or into mercaptans, orelse to hydrogenolyze at least partially these unsaturatedsulfur-containing compounds to form H₂S, is observed.

[0137] In the process according to this preferred embodiment of theinvention, the conversion of the unsaturated sulfur-containing compoundsis greater than 15% and preferably greater than 50%. In the same step,the hydrogenation rate of the olefins is preferably less than 50%, morepreferably less than 40%, and very preferably less than 35%, during thisstage. The effluent that is obtained from this first hydrogenolysisstage is then sent, preferably without any separation of liquid and gas,to stage f), which makes it possible to decompose the saturatedsulfur-containing compounds into H₂S.

[0138] Decomposition of the Sulfur-Containing Compounds that areContained in the Product that Originates from Stage e) (Stage f):

[0139] In this stage, the saturated sulfur compounds are transformed inthe presence of hydrogen on a suitable catalyst. The decomposition ofthe unsaturated compounds that are not hydrogenated during stage e) canalso take place simultaneously. This transformation is carried out,without significant hydrogenation of the olefins, i.e., that during thisstage, the hydrogenation of the olefins is generally limited to 20% byvolume relative to the olefin content of the starting gasoline, andpreferably, limited to 10% by volume relative to the olefin content ofthe starting gasoline.

[0140] The catalysts that are suitable for this stage of the processaccording to the invention, without this list being limiting, arecatalysts that generally comprise at least one basic element that isselected from among the elements of group VIII and group VIB, andpreferably selected from the group that is formed by nickel, cobalt,iron, molybdenum, and tungsten. These metals can be used alone or incombination; they are preferably supported and used in their sulfurizedform. The catalyst of stage f) preferably has a nature and/orcomposition different from the one used in stage e). The base metalcontent of the catalyst according to the invention is generally betweenabout 1 and about 60% by weight, preferably between 5 and 30% by weight,and very preferably between 10 and 25% by weight. Preferably, thecatalyst is generally shaped, preferably in the form of balls, pellets,extrudates, for example trilobes. The metal can be incorporated into thecatalyst by deposition on the preformed substrate; it can also be mixedwith the substrate before the shaping stage. The metal is generallyintroduced in the form of a precursor salt, generally water-soluble,such as, for example, the nitrates and the heptamolybdates. This methodof introduction is not specific to the invention. Any other method ofintroduction that is known to one skilled in the art may be suitable. Acatalyst that contains at least one element of group VIII, and inparticular nickel, is very advantageously used.

[0141] The substrates of catalysts that are used in this stage of theprocess according to the invention are generally porous solids that areselected from among the refractory oxides, such as, for example, thealuminas, silicas and silica-aluminas, magnesia, as well as titaniumoxide and zinc oxide, whereby these latter oxides can be used alone ormixed with alumina or silica-alumina. The substrates are preferablytransition aluminas or silicas whose specific surface area is 25-350m²/g. The natural compounds, such as, for example, the diatomaceousearth or the kaolin, can also be suitable as substrates of catalyststhat are used in this stage of the process.

[0142] The catalyst is preferably used at least in part in itssulfurized form. This offers the advantage of limiting as much aspossible the hydrogenation risks of the unsaturated compounds such asthe olefins or the aromatic compounds during the start-up phase. Thesulfurization stage can be carried out by any technique that is known toone skilled in the art, in situ or ex situ.

[0143] After sulfurization, the sulfur content of the catalyst isgenerally between 0.5 and 25% by weight, preferably between 4 and 20% byweight and very preferably between 4 and 10% by weight. The purpose ofthe hydrodesulfurization that is carried out during this stage is toconvert into H₂S the saturated sulfur-containing compounds of thegasoline that already underwent at least one preliminary hydrogenationof the unsaturated compounds of sulfur during stage e). It makes itpossible to obtain an effluent that meets the desired specifications interms of content of sulfur-containing compounds. The gasoline that isthus obtained exhibits only a small octane loss. The treatment that aimsat decomposing the saturated sulfur-containing compounds that originatefrom stage e) of the process is carried out in the presence of hydrogen,with the catalyst that comprises at least one base metal that isselected from the group that is formed by nickel, cobalt, iron,molybdenum, tungsten, at a temperature of between about 280° C. andabout 400° C., preferably between about 290° C. and about 380° C., morepreferably between 310° C and 360° C., and very preferably between 320°C. and 350° C., under a pressure that is generally between about 0.5 andabout 5 MPa, preferably between 1 and 3 MPa, more preferably between 1.5and 3 MPa. The volumetric flow rate of the liquid is between about 0.5and about 10 h⁻¹ (expressed by volume of liquid per volume of catalystand per hour), preferably between 1 and 8 h⁻¹. The H₂/HC ratio isadjusted based on the desired hydrodesulfurization rates in the range ofbetween about 100 and about 600 liters per liter, preferably between 100and 300 liters per liter. All or part of this hydrogen can optionally beobtained from stage e) (unconverted hydrogen) or a recycling of theunconsumed hydrogen in stages a), b) or c). It was found that theimplementation of this second catalyst in this stage, under particularoperating conditions, makes it possible to decompose the saturatedcompounds, contained in the effluent that originates from stage c, intoH2S. This implementation makes it possible to reach a high overall levelof hydrodesulfurization at the end of all of the stages of the processaccording to the invention, while minimizing the octane loss thatresults from the saturation of the olefins, because the conversion ofthe olefins during stage e) is generally limited to at most 20% byvolume of the olefins, preferably at 10% by volume.

[0144] In a particular embodiment, according to the characteristics ofthe starting gasoline feedstock, the latter is freed of the majority ofits basic nitrogen-containing compounds that are at least in parteliminated before stage c) for alkylation of at least a portion of thesulfur-containing compounds that are present in the product thatoriginates from stage b). According to a preferred embodiment, the basicnitrogen-containing compounds that are contained in the startinggasoline are at least in part eliminated before its introduction intostage a) for hydrogenation of the polyunsaturated compounds. Most often,the elimination of the basic nitrogen-containing compounds is carriedout by a treatment (washing) with an acid aqueous solution. Thus, whenthe starting gasoline contains basic nitrogen-containing compounds, thelatter are at least in part eliminated by a treatment with the help ofan acid aqueous solution that is carried out before stage c) foralkylation of at least a portion of the sulfur-containing compounds thatare present in the product that originates from stage b). This washingis usually carried out before or after the selective hydrogenationtreatment in stage a) of the polyunsaturated compounds that arecontained in the starting gasoline.

[0145] The catalysts that are used in stages e) and f) are most oftenseparate sulfurized catalysts.

[0146] Description of the Single Figure (which diagrammaticallyillustrates, in a nonlimiting manner, an embodiment of the invention)

[0147] Gasoline (a) that contains sulfur-containing compounds, diolefinsand olefins is injected into line (1). The hydrogen is injected intoline (2) in an amount such that the hydrogen/diolefin molar ratio isgreater than 1. The gasoline and the hydrogen are brought into contactin a reactor (A) for selective hydrogenation of diolefins, underoptimized conditions for limiting the saturation of olefins whilehydrogenating the diolefins.

[0148] The effluent of reactor (A) is sent via line (3) into a unit forincreasing the weight of sulfur-containing compounds (B). The reactionsthat are implemented in this reactor (B) are essentially reactions forincreasing the weight of mercaptans from 1 to 6 carbon atoms as well assulfides with 2 to 6 carbon atoms. A partial conversion of the compoundssuch as the CS2 and COS is also observed. The gasoline that is producedis introduced via line (4) into a reactor (C) for increasing the weightof sulfur-containing compounds by addition to the olefins. Thesulfur-containing compounds that are primarily increased in weight inthis reactor are the thiophenic compounds. Also in this reactor (C),oligomerization reactions of olefins and partial alkylation of benzenecompounds are observed. The gasoline that is produced in this reactor(C) is therefore both heavier than gasoline (a) and depleted of lightsulfur-containing compounds. The gasoline that is produced in reactor(C) is injected via line (5) into a fractionation column (D) thatseparates the gasoline into at least two fractions.

[0149] The light gasoline whose end point can be between 55° C. and 160°C. is recovered at the top of the column. This gasoline is desulfurizedand does not require additional treatment. The final temperature of thislight gasoline is set by the maximum amount of sulfur allowed.

[0150] The heavy gasoline that is recovered at the bottom of the columnvia line (7) is sent after mixing with the hydrogen that is introducedvia line (8) to a desulfurization section (E+F). This gasoline has adistillation starting point between 50° C. and 130° C.

[0151] Hydrodesulfurization section (E+F) is designed to desulfurize thegasoline while limiting the hydrogenation of olefins, which makes itpossible to limit the octane loss. It consists of at least two reactorsin series, the first (E) of which comprises an optimized catalyticsystem for saturating the thiophenic compounds and partiallytransforming the sulfur-containing compounds into H2S. The secondcontains a catalyst that is an optimized catalyst for transforming themercaptans into H2S, by limiting the hydrogenation of olefins that arepresent in the gasoline. This desulfurization scheme has been describedin Patent Application EP 1 077 247.

[0152] The heavy gasoline that is recovered via line (10) and the lightgasoline that is recovered via line (6) can be mixed to produce acomplete desulfurized gasoline that is recovered via line (11).

EXAMPLE 1

[0153] (For Comparison)

[0154] A cracking gasoline is subjected to a hydrogenation treatment ofthe diolefins under conditions where the saturated sulfur-containinglight compounds that are present in the feedstock are partly convertedinto heavier compounds.

[0155] This treatment is carried out in a continuously-operatingreactor. The catalyst has a nickel and molybdenum base (catalystmarketed by the Procatalyse Company under reference HR945). The reactionis performed at 180° C. under a total pressure of 2.6 MPa, with avolumetric flow rate of 6 h⁻¹. The H₂/feedstock ratio, expressed inliter of hydrogen per liter of feedstock is 10.

[0156] The characteristics of the catalytic cracking gasoline and theeffluent after hydrogenation of diolefins and conversion of lightcompounds are indicated in Table 1. TABLE 1 Gasoline after HydrogenationStarting Gasoline (Stages a and b) Density 15/4 0.7215 0.7237 BromineNumber 78 74 (gBr/100 g) Olefins (GC) % by 43 40.5 weight MAV (mg/g) 100.2 Research Octane 93 92.5 Number Motor Octane Number 79.6 79.4Mercaptans (ppm) 26 4 S Total (ppm) 350 350 Fraction Points (DS)  0.5% 22   5% 23 24   10% 30 31   50% 86 87   90% 141 143   95% 152 151 99.5%175 175

[0157] At the end of this treatment, the gasoline is separated into twofractions: a light fraction that represents 65% by weight of thedistilled gasoline whose fraction point corresponds to a temperature of100oC and a heavy fraction. The separation is carried out on abatch-type distillation column that consists of 30 theoretical plateaus.The characteristics of the two fractions that are obtained are providedin Table 2. TABLE 2 Light Fraction Heavy Fraction PI-100 100-175 Density15/4 0.6826 0.7712 Bromine Number 91 48 (gBr/100 g) MAV (mg/g) <0.2 <0.2Research Octane 93.4 91.8 Number Motor Octane Number 79.9 79 Mercaptans(ppm) 3 4 S Total (ppm) 62 885 Fraction Points (DS)  0.5% 4 93   5% 2099   10% 22 108   50% 60 128   90% 96 145   95% 99 152 99.5% 110 177

[0158] The light fraction exhibits contents of diolefins, mercaptans andsulfur such that it can be used directly, provided that thespecification on the sulfur content is greater than 60 ppm.

[0159] The heavy gasoline should be desulfurized before use. In all ofthe cases, this scheme does not make it possible to produce adesulfurized gasoline that contains less than 40 ppm of sulfur byrecombining the light gasoline and the heavy gasoline.

EXAMPLE 2

[0160] (For Comparison)

[0161] The catalytic cracking gasoline that is obtained in Example 1after hydrogenation treatment is separated into two fractions, lightfraction that represents 20% by weight of distilled gasoline whosefraction point corresponds to a temperature of 55° C. and a heavyfraction. The separation is carried out on the same column as inExample 1. The characteristics of the two fractions that are obtainedare provided in Table 3. TABLE 3 Light Fraction Heavy Fraction PI-5555-175 Density 15/4 0.65 0.77 Bromine Number 130 65 (gBr/100 g) Olefins(GC) 60 36 MAV (mg/g) <0.2 <0.2 Research Octane 95 90.5 Number MotorOctane Number 81.5 80 Mercaptans (ppm) <1 4 S Total (ppm) 2 437 FractionPoints (DS)  0.5% 4 52   5% 20 54   10% 22 67   50% 36 102   90% 41 138  95% 54 151 99.5% 72 176

[0162] The light gasoline that is produced by distillation exhibitscontents of mercaptans, diolefins and sulfur such that it can be useddirectly.

[0163] The heavy gasoline requires an additional desulfurization.

[0164] The heavy gasoline is therefore subjected to ahydrodesulfurization on a scheme of catalysts in an isothermal tubularreactor.

[0165] The first catalyst (catalyst A) is obtained by impregnation“without excess solution” of a transition alumina that comes in the formof balls with a specific surface area of 130 m²/g and a pore volume of0.9 ml/g, by an aqueous solution that contains molybdenum and cobalt inthe form of ammonium heptamolybdate and cobalt nitrate. The catalyst isthen dried and calcined under air at 500° C. The content of cobalt andmolybdenum of this sample is 3% of CoO and 14% of MoO3.

[0166] The second catalyst (catalyst B) is prepared from a transitionalumina of 140 m²/g that comes in the form of balls that are 2 mm indiameter. The pore volume is 1 ml/g of substrate. A kilogram ofsubstrate is impregnated by 1 liter of nickel nitrate solution. Thecatalyst is then dried at 120° C. and calcined under a stream of air at400° C. for one hour. The nickel content of the catalyst is 20% byweight. 100 ml of catalyst A and 200 ml of catalyst B are placed in tworeactors in series, so that the feedstock that is to be treated (heavyfraction) first encounters catalyst A, then catalyst B. A zone forsampling the effluent that originates from stage e is provided betweencatalysts A and B. The catalysts are first sulfurized by treatment for 4hours under a pressure of 3.4 MPa at 350° C., upon contact with afeedstock that contains 2% by weight of sulfur in the form of dimethyldisulfide in n-heptane.

[0167] The operating conditions of hydrodesulfurization are as follows:VVH=1.33 h⁻¹ relative to a complete catalyst bed H₂/HC=300 1/1, P=2.0MPa. The temperature of the catalytic zone that comprises catalyst A is280° C. whereby the temperature of the catalytic zone that containscatalyst B is 330° C.

[0168] The characteristics of the effluents that are thus obtained arepresented in Table 4. TABLE 4 Mixture of the Starting Desulfurized LightGasoline and Heavy Heavy the Desulfurized Fraction Fraction HeavyGasoline 55-175 55-175 PI-175 Density 15/4 0.7732 0.7696 0.7228 BromineNumber 65 38.5 56.2 (gBr/100 g) Olefins (GC) 36 25 32 % by Weight MAV(mg/g) <0.2 <0.2 <0.2 Research Octane 90.5 85.4 88.9 Number Motor OctaneNumber 80 76 78.2 Mercaptans (ppm) 4 12 10.4 S Total (ppm) 437 30 25Fraction Points (DS)  0.5% 52 2   5% 54 23   10% 67 30   50% 102 86  90% 138 141   95% 151 152 99.5% 176 175

[0169] This scheme as described in Patent Application EP 1 077 247 makesit possible to produce a desulfurized gasoline whose contents ofmercaptans and diolefins are compatible with the grades that arerequired for the gasolines.

[0170] In the particular case of this example, the desulfurization rateis 92.8%, the residual sulfur content is 25 ppm and the octane loss thatis calculated by the formula (RON+MON)/2 is 2.75 points.

EXAMPLE 3

[0171] (According to the Invention)

[0172] The catalytic cracking gasoline that is obtained in Example 1after hydrogenation treatment [stages a) and b) according to theinvention] is sent to a reactor for increasing the weight ofsulfur-containing compounds by alkylation by the olefins (stage c)according to the invention). This stage is implemented in a tubularreactor that contains catalyst with a phosphoric acid base that issupported on silica that contains 20% by weight of phosphoric acid(catalyst C) under the following operating conditions: VVH=1h-1,pressure=2.0 MPa, temperature=180° C. Before injection in the reactor,the feedstock is supplemented with isopropanol at a level of 500 ppm,intended to hydrate the catalyst continuously in the reactor.

[0173] The effluent that is thus produced is separated into twofractions with a distillation column as described in Example 1. Thedistillation fraction point is set at 100° C; the light fractionrepresents 50% by weight of the starting gasoline.

[0174] The characteristics of the gasoline produced during alkylationstage c) and fractionation stage d) are provided in Table 5. TABLE 5Gasoline after Being Light Heavy Increased in Weight by FractionFraction Alkylation (Stage c) (Stage d) (Stage d) PI-240 PI-100 100-240Density 15/4 0.7641 0.6921 0.8124 Bromine Number 62 53 72 (gBr/100 g)MAV (mg/g) <0.2 <0.2 <0.2 Research Octane 91.4 86.2 93.6 Number MotorOctane Number 80.7 79.5 81.7 Mercaptans (ppm) 5 4 S Total (ppm) 350 18670 Fraction Points (DS)  0.5% 6 6 94   5% 23 19 102   10% 30 24 108  50% 105 61 154   90% 206 97 210   95% 215 100 219 99.5% 240 111 238

[0175] The light gasoline that is recovered at the outlet of thefractionation stage-exhibits total contents of diolefins, mercaptans andsulfur such that it can be used without additional treatment. The heavygasoline requires a desulfurization stage.

[0176] The desulfurization of the heavy gasoline (stages e) and f) ofthe process according to the invention) is carried out with the devicethat is described in Example 2. The operating conditions are as follows:VVH=1.33 h⁻¹ relative to a complete catalyst bed H₂/HC=300 1/1, P=2.0MPa. The temperature of the catalytic zone that comprises catalyst A is290° C; the temperature of the catalytic zone that contains catalyst Bis 340° C.

[0177] The gasoline that is thus produced exhibits no more than a sulfurcontent of 26 ppm. It can be used without additional treatment. Thisgasoline is recombined with the light gasoline that is recovered instage d).

[0178] The characteristics of the desulfurized heavy gasoline and therecombined gasoline are provided in Table 6. TABLE 6 DesulfurizedRecombined Heavy Light Light Gasoline Gasoline Starting Gasoline (Stagese) and Heavy Gasoline (Stage d) and f) Gasoline PI-175 PI-100 100-240PI-240 Density 15/4 0.7215 0.6921 0.8134 0.7683 Bromine Number 80 53 3745 (gBr/100 g) Olefins (GC) 44 % by Weight MAV (mg/g) 12 <0.2 <0.2 <0.2Research Octane 93 86.2 90.4 88.8 Number Motor Octane 79.8 79.5 80.279.7 Number Mercaptans (ppm) 20 4 12 9 Sulfur, Not 330 14 24 17Mercaptan (ppm) S Total (ppm) 350 18 36 26

[0179] This scheme that is carried out according to the invention makesit possible to produce a desulfurized gasoline with a limited octaneloss, whose contents of mercaptan and diolefins are compatible with thegrades that are required for the gasolines.

[0180] In the particular case of this example, the desulfurization rateis 92.6%; the residual sulfur content is 26 ppm, and the octane lossthat is calculated by formula (RON+MON/2) is 2.15 points.

1. Process for the production of gasoline with a low sulfur content froma starting gasoline that contains at least 150 ppm by weight ofsulfur-containing compounds comprising at least the following stages: astage a) for selective hydrogenation of non-aromatic, polyunsaturatedcompounds that are present in the starting gasoline, at least one stageb) that aims at increasing the molecular weight of the lightsulfur-containing products, primarily those that are in the form ofmercaptans that have 1 to 6 carbon atoms in their molecules and sulfidesthat are initially present in the gasoline that is introduced in stagea) and/or those that are contained in the product that originates fromstage a), at least one stage c) for alkylation of at least a portion ofthe sulfur-containing compounds, primarily those that are in the form ofthiophenic compounds that are present in the product that originatesfrom stage b) that aims at obtaining sulfur-containing compounds with ahigher molecular weight, at least one stage d) for fractionation of thegasoline that originates from stage c) into at least two fractions, afirst fraction that is virtually lacking in sulfur and that contains thelightest olefins of the unconverted starting gasoline in stage c),(light gasoline), at least one other fraction, heavier than said firstfraction, enriched with sulfur-containing compounds, and at least onestage e) for treatment of at least one of the heavier fractions thatoriginates from stage d) on a catalyst that makes it possible todecompose at least partially the sulfur-containing compounds.
 2. Processaccording to claim 1, in which stage e) for treatment of at least one ofthe heavier fractions that is separated in stage d) on a catalyst thatmakes it possible to decompose at least partially the sulfur-containingcompounds is carried out under conditions where the hydrogenation ofolefins on this catalyst is limited.
 3. Process according to claim 1 or2 comprising at least one stage f) for treatment of the product that isobtained in stage e), without elimination of the H₂S that is formedduring this stage e), on a catalyst and under conditions that make itpossible to decompose at least partially the sulfur-containing compoundsthat are not transformed during stage e) with a limited hydrogenation ofolefins.
 4. Process according to one of claims 1 to 3, in which thestarting gasoline is a catalytic cracking gasoline whose final boilingpoints are from about 120° C. to about 230° C.
 5. Process according toclaim 3 or 4, in which the conditions of stage f) are selected so as todecompose the unsaturated sulfur-containing compounds and the linearand/or cyclic saturated sulfur-containing compounds that are nottransformed during stage e), with a limited hydrogenation of olefins. 6.Process according to one of claims 1 to 5, in which stage a) forhydrogenation of unsaturated compounds and stage b) that aims atincreasing the molecular weight of the light sulfur-containing productsthat are initially present in the gasoline that is introduced in stagea) are carried out simultaneously in a single reaction zone thatcontains one or more beds of a single catalyst.
 7. Process according toone of claims 1 to 6, in which the starting gasoline contains basicnitrogen-containing compounds that are at least in part eliminatedbefore stage c) for alkylation of at least a portion of thesulfur-containing compounds that are present in the product thatoriginates from stage b).
 8. Process according to claim 7, in which thebasic nitrogen-containing compounds that are contained in the startinggasoline are at least partly eliminated before its introduction intostage a) for hydrogenation of the polyunsaturated compounds.
 9. Processaccording to claim 6 or 7, in which the elimination of the basicnitrogen-containing compounds is carried out by a treatment with an acidaqueous solution.
 10. Process according to one of claims 3 to 9, inwhich the desulfurized heavy gasoline that originates from stage f) issubjected to a stripping treatment by means of a cover gas.
 11. Processaccording to one of claims 3 to 10, in which stages e) and f) arecarried out in at least two successive and separate reaction zones. 12.Process according to one of claims 3 to 11, in which the light gasolinethat originates from stage d) and at least a portion of the heavygasoline that originates from stage f) are mixed to form the overalldesulfurized gasoline that is desired.
 13. Process according to one ofclaims 1 to 12, in which the catalyst of stage e) comprises at least oneelement of group VIII and at least one element of group VIB.
 14. Processaccording to one of claims 3 to 13, in which the catalyst of stage f)comprises at least one element of group VIII.
 15. Process according toone of claims 3 to 14, in which the catalysts that are used in stages e)and f) are separate sulfurized catalysts.
 16. Process according to oneof claims 3 to 15, in which stages e) and f) are carried out in tworeactors that are placed in series, whereby the second reactor treatsthe effluent of the first reactor as a whole.
 17. Process according toone of claims 1 to 16, in which alkylation stage c) is carried out underconditions such that a portion of the olefins of the starting gasolineis converted into long branched olefins by addition reactions betweenolefins and such that a portion of the aromatic compounds is increasedin weight by alkylation by the olefins.
 18. Process according to one ofclaims 1 to 17, in which the heavy fraction that originates from staged) is sent into a fractionation zone that makes it possible to obtain aheavy fraction that is enriched with sulfur and a lighter fraction thatis low in sulfur.
 19. Process according to one of claims 1 to 17, inwhich the gasoline that originates from hydrodesulfurization stage e) orf) is sent into a fractionation zone that makes it possible to obtain aheavy fraction that is enriched with sulfur and a lighter fraction thatis low in sulfur.